I have looked into this and I have found a USB to VGA ( http://www.sitecom.com/products_info.php?product_id=292&grp_id=1 ) which allows fro a second monitor to be connected through the USB 2.0 port. Unfortunately it doesn't seen to have Mac OS X drivers and I don't think that it would be plug and play for OS X. I am going to continue to look for a driver hack that would allow it to be used with the Mac.

I am puzzled by the message that I saw in the ATI Displays 4.5 Update,
My Mac mini screen system profiler gives a message for the ATY, RV280 as:-
Display:
Etc.,
Main Display: Yes
Mirror: off
Online: Yes

Now no where can I make sense of that from the block diagram of the Mac mini until you look at Page 32 it talks of the svideo device following the DVI Port and adaptor in the Mac mini developer note.
When I spoke to our local Tech he said that the signal from the Mac mini would be too weak but I am still looking for a solution not a hack but some plug and play way to have two outputs. I would like to have the playability for teaching and other uses so any suggestions other than hacking I am interested in.